Generally, static electricity means tribo-electrification generated by rubbing two different objects, or peeling static electrification generated by separating two adhered objects. Static electricity can cause suction of foreign particles such as dust, electrostatic destruction of a device, malfunction of a measuring instrument, fire, etc. Specifically, the demand of liquid crystal display (LCD) has been remarkably increased due to wide spread of a mobile type of computer, expansion of liquid crystal TV or mobile phone having multi function, etc. As each assembly device is integrated and becomes vulnerable to static electricity, it is more important to inhibit generation of static electricity.
By the increased demand of LCD, the demand of optical films such as polarizer has been rapidly increased, thereby requiring high speediness of the LCD preparation process. Also, up to now, major LCD makers have concentrated on producing small panels less than 20 inches, but now expanding rapidly the production range to large sizes over 20 inches thanks to high technology.
Particularly, the size of polarizer used for the LCD production becomes larger, as the LCD size becomes larger. And, as the LCD preparation process becomes speedier, static electricity is excessively generated when the releasing film is peeled. Such generated static electricity affects the orientation of liquid crystal in the LCD to cause image defect.
Nowadays, the surface treatment of polarizer is done in various ways. The surface treatment of polarizer includes anti-glare layer, low and/or high reflection layer, anti-fouling layer, etc., and each layer has different surface roughness and surface energy. Particularly, the anti-glare layer has higher surface roughness and lower surface energy than triacetylcellulose (TAC) layer, and so the wetting property of the pressure sensitive adhesive is important when a protective film is adhered thereto. Also, as the preparation process of polarizer is conducted in high speed, TFT IC device causing malfunction of display panel may be destroyed by static electricity which is not generated in the conventional process, at the time of peeling the protective film of polarizer.
In order to prevent generation of static electricity as explained above, a method of forming anti-static layer on an exterior surface of polarizer or protective film was proposed, but the effect was not good enough, and could not prevent generation of static electricity basically. Therefore, the anti-static function is needed for the pressure sensitive adhesive to prevent generation of static electricity completely.
Conventional methods for providing the anti-static function of the pressure sensitive adhesive include a method of adding conductive components such as conductive metal powder or carbon particle, or a method of adding a surfactant type of ionic or anionic material. However, the method of adding metal powder or carbon particle had a disadvantage that transparency of the pressure sensitive adhesive layer is decreased since a large amount of metal powder or carbon particle should be used to provide the anti-static function. Also, the method of adding a surfactant type of ionic or anionic material had disadvantages that it is vulnerable to humidity, and adhesive property is decreased due to the migration of surfactant to a surface of the pressure sensitive adhesive.
Also, a method of discharging static electricity in the generation process by applying static eliminator has been proposed, but has a disadvantage to decrease the productivity due to limitation of the discharging capacity and lowering of the process speed.
Also, Japanese Patent Publication No. 1993-140519 discloses a method for preventing generation of static electricity by adding plasticizer of ethyleneoxide-modified phtalic acid dioctyl in the pressure sensitive adhesive. However, the method has disadvantages that the plasticizer is migrated to the surface of polarizer, and it is difficult to inhibit static electricity generated in the beginning.
Korean Patent Publication No. 2004-0030919 discloses a method of preparing an antistatic pressure sensitive adhesive comprising at least about 5% by weight of the organic salt to have the surface resistivity of less than about 1013 Ω/□. However, this method has disadvantages that expensive organic salt should be used, and the change of constant-voltage generated in the peeling process cannot be predicted since the method decreases only the surface resistivity.
Japanese Patent Publication No. 2004-287199 discloses a method for providing anti-static property by adding ion-conductive polymer having carboxyl group. However, the method has disadvantages that the ion-conductive polymer react with a conventional crosslinking agent (isocyanate) to change the adhesive and rheology property, and thus it is difficult to control the anti-static property and adhesive strength.
Also, Japanese Patent Publication No. 1994-128539 discloses a method for providing anti-static property by mixing polyether polyol compounds and one or more alkali metal salts. However, this method has disadvantages that when the crosslinking agent is isocyanate, the use of polyether polyol compounds may affect the degree of crosslinking, and the adhesive property may be decreased by ether linkage, i.e., the migration due to the hydrophilic property of ethyleneoxide.